Satellite radiotelephone systems are being developed and deployed at many locations around the world. Satellite radiotelephone systems are often used where rugged terrain or lack of population density may preclude use of conventional mobile radiotelephones or fixed (wire) telephones. The design and operation of satellite radiotelephone systems are well known to those having skill in the art and need not be described further herein. It will be understood that radiotelephone systems can be used for voice and data communications, so that radiotelephones can include terminals such as PCS terminals.
Satellite radiotelephone systems may be broadly classified into two types: mobile satellite radiotelephone systems and fixed satellite radiotelephone systems. In both types of systems, one or more satellites are used to communicate with radiotelephones. The satellites may be orbiting satellites or geostationary satellites. A mobile satellite radiotelephone system is designed to communicate with a plurality of mobile radiotelephones of similar size as conventional cellular radiotelephones. In contrast, a fixed satellite radiotelephone system is designed to communicate with a plurality of fixed (nonmobile) radiotelephones using permanent or semi-permanent fixed antennas which may be mounted on buildings or homes.
The capacity of fixed satellite radiotelephone systems is generally relatively large. The term "capacity" is used to refer to the number of radiotelephones with which the satellite radiotelephone system can communicate simultaneously.
The per satellite capacity of a satellite radiotelephone system is generally limited by the amount of satellite power that is expended per communications circuit in order to establish and maintain communications with a radiotelephone. Another limiting factor of satellite radiotelephone system capacity may be the available frequency spectrum and the typically poor frequency reuse of satellite radiotelephone systems.
Mobile satellite radiotelephone systems generally have a much lower capacity than fixed satellite radiotelephone systems. In particular, the regional and global mobile satellite systems that are currently under development are generally quite limited in capacity. Regional mobile satellite systems involving geostationary satellites may have a capacity per satellite of about 10,000 simultaneous radiotelephone communications. Global mobile satellite systems involving medium earth orbiting (MEO) or low earth orbiting (LEO) satellites generally have even lower capacities per satellite, typically three to four thousand simultaneous radiotelephone communications.
A primary limitation in the per satellite capacity of mobile satellite radiotelephone systems is the amount of power that is expended per communication by the satellite payload in order to establish and maintain communications with the small hand-held mobile phones. Moreover, the practical limitations of forming a very large number of spot beams from the satellite often limits frequency reuse for mobile satellite radiotelephone systems. For these and other reasons, the capacity of mobile satellite radiotelephone systems is generally low.
Fixed satellite radiotelephone systems generally do not include mobile telephone services. Rather, services are delivered to end user homes or businesses via permanently or semi-permanently fixed terminal installations. These installations generally include directional antennas that in some cases are capable of tracking the satellites.
Fixed satellite radiotelephone systems generally have higher capacity than mobile systems. The fixed satellite radiotelephone systems communicate using fixed user terminals which can use relatively large end user antennas. Hence, the satellite power required per equivalent communication can be lower for a fixed system than for a mobile system. Furthermore, frequency allocations for the fixed systems can be more liberal than those of mobile systems for at least two reasons. First, fixed systems generally operate at higher frequencies such as C-band or above, where the frequency spectrum is not as crowded. Second, the user terminals are fixed and use highly directional antennas so that angular separation between satellites can be relied upon for frequency reuse.
Due to the relatively limited capacity of mobile satellite radiotelephone systems, capacity bottlenecks or "hot spots" may develop in congested areas of the mobile satellite radiotelephone system, where the mobile satellite radiotelephone system does not have enough capacity to accommodate all users. It may be difficult to increase the capacity of the mobile satellite system in these congested areas.